Magnetic disk devices with a plurality of independently-driven actuators, for example, with two actuators (dual actuators) are proposed. In such a magnetic disk device, two actuators are vertically and coaxially separated such that the actuators can separately read/write data from/to a magnetic disk. With such a structure, the random access performance of the magnetic disk device to magnetic disks can be improved.
In some cases, magnetic disk devices are suddenly shut down. In that case, in order to protect the magnetic disks, heads must be retracted from the magnetic disk. For example, if the power is cut, a back electromotive force of a spindle motor which operates an actuator with the head is used to supply power to a controller configured to control the actuator and the actuator is retracted. Furthermore, the back electromotive force of the spindle motor is used as the power of internal logic circuit or the like of a motor driver.
Now, a relationship between a retract power and a back electromotive force in a magnetic disk device including one actuator (single actuator) will be explained. If the power is cut, brake current is supplied to stop the operation of the single actuator. The back electromotive force decreases in proportion to the current because of a coil resistance of a spindle motor. Then, after the actuator with the head is moved to an outer periphery part of the magnetic disk at a constant speed, a great force is generated in a ramp direction because of a magnet latch. Current to perform the retraction is supplied to cancel the force; however, the current to perform the retraction is limited since the internal logic circuit and the like of a motor driver do not properly work when the back electromotive force becomes too low.
In contrast, in the magnetic disk device of dual actuator, current to perform the retraction must be doubled as compared to the magnetic disk device of single actuator. Thus, if the power is cut and the dual actuators are retracted but the back electromotive force is significantly decreased and the current to perform the retraction is limited, sufficient current to perform the retraction of two actuators may not be obtained. When the current to perform the retraction is insufficient, the actuators each collide an outer stop at a fast speed and contaminants may be produced which may decrease the reliability of the magnetic disk device.
The present application presents safety retraction control of a plurality of actuators driving independently and a magnetic disk device adopting the same control to improve the reliability.